Interpretive Summary: Infants born prematurely are smaller than full-term infants and can have frail bones. This is because mineralization of bone is highest in the third trimester (last 3 months) of pregnancy. Despite advances in the formulas developed for preterm infants to promote growth, the changes in bone are often slower than if the infant had not been born prematurely. Hence, premature infants have a risk of bone fractures. Over the past 25 years, several techniques have been developed to assess bone density in adults and children, but none of these methods is feasible for use in sick or premature infants. We have examined a new technique, based on the speed of sound, as a potential means of assessing the quality of bone of preterm infants. Measurements in the arms and legs of 40 infants of different ages, all of whom had been born prematurely, were made. We found that the measurements could be accurately reproduced, the values increased with age, and were slightly different between the arm and leg which may indicate differences in bone growth patterns. Since the technique was well tolerated by these relatively healthy infants, future studies will examine smaller and sicker preterm infants.

Technical Abstract:
Our objective was to measure bone speed of sound in relatively healthy preterm infants to assess its reproducibility, its relationship with post-menstrual age and age at which full enteral feeds were attained. A cross-sectional study of 40 (24 male, 16 female) non-ventilated premature infants at a postmenstrual age of less than 36 weeks at birth. Speed of sound measurements (Sunlight Omnisense 7000P instrument) were obtained on one occasion at the tibial midshaft in these preterm infants once they attained a full feeding volume of at least 100 ml/kg/day. Measurement reproducibility was 0.8 ± 0.6% for repeat speed of sound measures in the same leg (n = 17). Differences in measurements were 1.4 ± 1.0 % for measurements made in both legs (n=39). Speed of sound significantly increased with postmenstrual age (y = 2384.7 + 17.25x; r2 = 0.128, p < 0.03) while there was no significant correlation with length of time taken to achieve full feeds. There were no differences between speed of sound measures in those infants whose mothers did or did not (p = 0.6) receive prenatal steroids. Tibial speed of sound was not significantly related to gender, race, or serum calcium, phosphorus or alkaline phosphatase activity. Speed of sound measurements are highly reproducible, increase with PMA, and the technique is well tolerated by relatively healthy preterm infants. No effects of early feeding or prenatal steroid use on SOS were seen.